1. Field of the Invention
The present invention relates to an image forming technique utilizing electrophotography, such as a printer, a copier machine and a facsimile machine.
2. Description of the Related Art
A conventional image forming apparatus technique utilizing electrophotography is known in which developer means makes toner adhere to an electrostatic latent image which is formed on a photosensitive member by exposure means, a toner image is accordingly formed and then transferred onto a transfer paper, and thus transferred toner image is fixed by fixing means on the transfer paper while the transfer paper is transported. Known in particular as an apparatus which permits to form a color image is an image forming apparatus in which a toner image which is formed on a photosensitive member is primarily transferred onto an intermediate transfer medium and thus transferred transfer image now on the intermediate transfer medium is secondarily transferred onto a transfer paper.
In this image forming apparatus, for the purpose of forming a color image, toner images in a plurality of colors are formed one after another on the photosensitive member while primarily transferring each toner image onto an intermediate transfer medium every time each toner image is formed, a color toner image, which is the toner images in the plurality of colors superimposed one atop the other, is accordingly formed on the intermediate transfer medium, and thus formed color toner image is secondarily transferred onto a transfer paper, whereby a color image is obtained. The image forming apparatus having such a structure is capable of printing in a single color using toner in a particular color among the plurality of colors, typically the black color.
Meanwhile, among image forming apparatuses already in a practical use is an image forming apparatus which permits to use, as a transfer paper, a thicker paper which is thicker than a plain paper, such as a wood-free paper, a postcard, an envelop, a glossy paper and a high definition paper, in addition to a plain paper. However, when fixing means operates while such a thick paper mentioned above is transported at the same speed as that for a plain paper, a sufficient fixing capability is not obtained and the image quality accordingly deteriorates. While it is possible to prevent such a situation by increasing the heat capacity of the fixing means for instance and improving the fixing capability, this solution leads to an increase in size of the apparatus or allows excessive heat to degrade the image quality during fixing on a plain paper. Hence, when a thick paper is used as a transfer paper, such an image forming apparatus slows down a paper transportation speed slower than the speed for a plain paper to thereby extend a fixing time and ensure a sufficiently fixing capability.
An apparatus noting this may be an apparatus which lowers not only a paper transportation speed but also a process speed during forming of an image on a photosensitive member, during primary transfer onto an intermediate transfer medium or during other process. Alternatively, such an apparatus may be an apparatus which slows down only a paper transportation speed but maintains a process speed set to a normal speed. In the former apparatus, since an image forming process is performed at two types of speed, there is a problem that it requires double the labor for calculation of favorable process settings regarding a developing condition, a transfer condition and the like than where the image forming process is performed at only one type of speed and that it therefore takes long time to design the apparatus and confirm operations. In the latter apparatus, however, since one type of speed is used during the image forming process, such a problem will not arise.
Yet, when the latter apparatus is used, it is necessary to sufficiently study the timing of switching from a normal speed to a low speed. While to synchronize the timing of switching to the end of development of a photosensitive member for instance is desirable since this allows to avoid an increase in the number of control parameters and prevent a control program from becoming complex, there is a danger that the throughput of image form could decrease depending on the timing at which development of the photosensitive member ends. For instance, in the case of an apparatus in which an intermediate transfer medium includes a transfer area, which permits primary transfer of a toner image, and a transfer protection area which prohibits primary transfer of a toner image and which is provided along a direction of rotation axis, image form control is performed while referring to one revolution of the intermediate transfer medium, if the switching timing is too early and a driving time for driving the intermediate transfer medium at a low speed consequently becomes long, the throughput of image form will decrease.
Further, in the case of an apparatus which performs image form control using one round of an intermediate transfer medium as a reference, if the switching timing from a normal speed to a low speed is too early, an operation time at the low speed will become long and the throughput of image form will decrease. In a configuration that there are N pieces of toner transfer areas within an intermediate transfer medium, i.e., the first through the N-th (where N is an integer which is equal to or larger than 2) toner transfer areas, a corresponding to predetermined transfer paper size arranged one next to the other along a direction of rotational driving, when there is only one toner image to be transferred, if this toner image is formed inside the first toner transfer area and the apparatus slows down in synchronization to the stop of developing means, it takes long time for a transfer paper to move passing the remaining (Nxe2x88x921) pieces of transfer areas and the throughput of image form will decrease.
In addition, when such an intermediate transfer medium is used, if a toner image transfer end position differs between different transfer paper sizes, the design of operations for forming of the next image becomes complex such as an elapsed time from the end of forming a toner image until an abutting operation during which a cleaning member or the like abuts on the intermediate transfer medium or a clearing operation during which the cleaning member or the like leaves the intermediate transfer medium, and it becomes necessary to execute elaborate debugging to design a control program. This leads to a problem that the number of design steps to design the control program increases and development of the apparatus takes long time.
A primary object of the present invention is to provide an image forming apparatus and an image forming method which suppress a decrease in throughput of image form in a configuration that there is a low speed mode which uses a low speed as a transfer paper transportation speed.
A different object of the present invention is to provide an image forming apparatus and an image forming method which make it easy to design the timing of forming an image in a configuration that there is a low speed mode which uses a low speed as a transfer paper transportation speed.
The present invention is directed to an image forming apparatus, comprising: developing means which makes toner adhere to an electrostatic latent image formed on a photosensitive member to thereby form a toner image; an intermediate transfer medium onto which the toner image is primarily transferred during rotations of the intermediate transfer medium; secondary transfer means which secondarily transfers the toner image thus primarily transferred and now on the intermediate transfer medium onto a transfer paper; and fixing means which fixes the toner image on the transfer paper while the transfer paper is transported, wherein the image forming apparatus comprises transportation control means which has a normal mode for transporting a transfer paper at a first speed and a low speed mode for transporting a transfer paper at a second speed which is slower than the first speed at least from a secondary transfer position to a fixing end position, the image forming apparatus comprises intermediate transfer medium control means which drives the intermediate transfer medium at the first speed in the normal mode, but drives the intermediate transfer medium at the first speed in the low speed mode at least until the end of forming a toner image performed by the developing means, reduces a driving speed of driving the intermediate transfer medium from first speed to the second speed after the end of forming the toner image and then maintains the second speed at least until the end of secondary transfer, the image forming apparatus comprises image form control means which controls image forming on the photosensitive member so as to change a primary transfer start position on the intermediate transfer medium in accordance with a transfer paper size, the intermediate transfer medium has a transfer area, in which primary transfer of a toner image is possible and which is preset from the downstream side toward the upstream side along a direction of rotational driving, and the image form control means, in the low speed mode, controls the image forming so that a toner image will be primarily transferred onto such a portion of the transfer area which is on the upstream side of the intermediate transfer medium along a direction of rotational driving.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for purpose of illustration only and is not intended as a definition of the limits of the invention.